Television has long been viewed as a one-way information service. Television programming is provided over a number of channels and broadcasts over a wide viewing area. The television signals may be provided through over-air transmission, coaxial cable, a combination of fiber and coaxial cables (known as hybrid fiber/coax), or via satellite. More recently, interest has developed in the ability to provide interactive or two-way communications through television. One method for providing interactive television services is to provide a home with a computer processor with graphical output capability (also known as multimedia processing). Such a processor may be made to modify a television screen to reflect a user's inputs to the system. Thus, an interactive television application can be run on the computer processor with the output of the application viewed on the television while computer server systems at the headend provides the underlying data. Such a system requires that an expensive computer system be placed at each home with further expensive high speed data communications modem technology to link the computer system to the data server computers back at the headend or central office.
Another option for providing interactive television service to homes is to provide the interactive processing ability at the headend of the broadband system with just its output sent to the home for viewing and user inputs from the home are sent to the headend to provide input to the headend system to control its output back to the home. An immediate advantage to locating the processing systems at the headend is that the operator only needs one processor for each active, or simultaneous, user. This contrasts the first method of placing a processor in each subscribers home whether it is used or not. With the processing placed in a central location such as a broadband system headend, an interactive television system could be built much like a telephone switching system. When a user wishes an interactive video service, a call is set-up where a user is assigned a processor card within the central switch and is also assigned a radio frequency (TV) channel between the central switch and that subscriber's home over which to view the interaction. Commands from the user's remote control in the home are relayed back to the headend and directed to the processor card serving that user. The output of the card is viewed by the user over his or her private channel on the user's television set. The systems resources, including the private channel, remain assigned to the individual user for the duration of use and, when the user tunes back to broadcast channels, these resources returned to an allocation pool available for another user.
In one paper, it has been suggested that interactive services be delivered to individual busses on a demand basis only. Large, D., "Tapped Fiber vs. Fiber-Reinforced Coaxial CATV Systems", IEEE LCS Magazine, February, 1990, pp. 12 et seq. A three level distributed switching system was proposed, with one switch at the headend to switch among hubs, one at each hub to switch among distribution lines, and a third level interdiction circuit to select the service for each dwelling. No architecture for such a scheme was proposed.
Van der Star, Jack A. M., "Video on Demand Without Compression: A Review of the Business Model, Regulation and Future Implication", discusses employing frequency re-use to distribute video-on-demand. The conventional cable trunk is split to produce separate feeds for each neighborhood. A video-on-demand block of frequencies is combined with the conventional frequencies. The video-on-demand frequencies are reused in each neighborhood. Each neighborhood trunk appears to be associated with a plurality of agile modulators equal to the number of video-on-demand channels. The article recommends that the number of video-on-demand channels equal 10% of the number of subscribers on the neighborhood trunk. The article further calls for an addressable decoder for each subscriber.